1. Field of the Invention
The present invention relates to a Radio Data System (RDS) signal detection device incorporated in a device to receive and demodulate RDS broadcasting signals, which judges whether signals received are RDS signals or not.
2. Description of the Prior Art
In an RDS broadcast mainly broadcast in Europe, automatic reception control of broadcasting stations which broadcast the same programs is performed using Alternative Frequency (AF) codes contained in data transmitted. In this case, it is necessary to search for a station having the best reception among stations listed in an AF code list and broadcasting the same program. It is also necessary to switch to the selected station as soon as possible. Therefore, selection of the best reception station has been conventionally performed by detecting the strength of reception electric field at each station while output sounds are muted for a very brief duration that is not uncomfortable for listeners so that a reception station is switched to an AF station.
In reception control as described above, the reception station selected according to the AF code can be a non-RDS station, or reception sensitivity of the station is sometimes not good enough to receive RDS data. In cases like these, a Program Identification code (PI code) for judging whether the station selected is a station to broadcast the same program is not detected, and as a consequence selection of a proper station is not performed. Therefore, re-selection of a station on the AF list having a second best strength of reception electric field is performed. For a quicker, more accurate selection, it is necessary to judge, as soon as possible, whether a station switched to is an RDS station or not. Likewise, if only RDS stations are sought and selected or memorized, it is necessary to judge, as soon as possible, whether the station selected is an RDS station or not.
A judging action of an RDS station such as that described above has conventionally been performed by detecting block synchronization or group synchronization of RDS data after broadcasting signal reception has been started by the selected station which was assumed to be an AF station.
The RDS data is configured in blocks of 26 bits. A certain synchronization pattern (offset words) is added to each block. Block synchronization is established by detecting the periodicity of the patterns. One group is configured in four blocks, and group synchronization is established by judging an order of offset words. Conventionally, at least 44 msec, which is equivalent to the length of two blocks, was necessary, since synchronization establishment of RDS data was used to judge an RDS station. Depending on reception conditions, about 200 msec is necessary for judgment because more than 100 msec is necessary for synchronization establishment. Therefore, switching of reception stations may be delayed, or listeners may feel uncomfortable due to a long muting period.